Exhaust backbox vs centre silencer removal.

[tom4motion]

As title really

Has anyone got any first hand knowledge of doing either or both to the standard exhaust? If so what affect did it have on volume and "noise"?

Cheers, Tom

[IanH755]

I removed both Back-Boxes but left the centre "resonators" fitted - I had flanges put on the back-boxes so that I could swap then back on if the noise was too loud, only a 10 min job to do.

With the back-boxes off the first 30 seconds of a "cold start" in a morning woke people 3 houses away who complained, no word of a lie!

Once it's warmed up past 30 seconds it's still loud and, personally, fantastic sounding but I'm a fan of loud brash car noise with no wife/kids who would be complaining about it. There was no motorway drone but the overall general noise level was raised slightly, until you floor it and it's gets very loud.

I had the work done by MIJ in Birmingham with some pictures below to show what they did and how it looked (tips deliberately chosen to look OEM rather than anything bigger). That was the 3rd time I had MIJ do exhaust work on my various cars and my second RS6 for a back-box removal. IMHO the C5 V8 I had done sounded less "anti-social" than the C6 V10 did







I also put a video on YT with the car at different throttle levels with an external mic which, to be brutally honest, was getting over powered by how loud the exhaust was so it's not great sounding at full throttle on the video even though in Real Life it sounded great -

YouTube Video - https://www.youtube.com/watch?v=yOCnJ_eYveE

The later 0-206mph video on my channel had a different 3in custom exhaust fitted and that had back boxes, tiny little shoe-box sized ones

[tom4motion]

Thanks for the detailed response Ian , im surprised at the difference removing the back boxes make.

Also MIJ not a million miles away from me either.

Cheers, Tom

[IanH755]

im surprised at the difference removing the back boxes make.

Hi Tom, the back-boxes are really the only "silencers" the car has as the centre-boxes are only "resonators" which take away drone (cancel resonance) but do very little to muffle the volume of the exhaust noise. So if you whip the back-boxes off you effectively have just 2 cats and a resonator per exhaust pipe to quieten the engine and thats it!

I chopped my C5's back-boxes off too and loved the noise but the volume difference is far louder on the newer C6 than the C5 ever was.

As I said, I loved the noise but no-one else did

[c3nturi0n]

other than cats in the downpipes, any other cats on C6s?

I'm thinking about replacing the OEM downpipes with aftermarket exhaust and wonder how to make it 'road legal' (so that it can pass the MOT emissions at least)?

[cammmy]

You will fail the MOT if you remove the cats in the downpipes. You may be able to fit higher flowing ones though.

viewtopic.php?f=47&t=134671

[c3nturi0n]

cheers cammy.

I know that many have not attempted this due to the complexity and other reasons mentioned in this thread. I wonder if downpipe replacement / swap /alteration is a doable task w/o dropping the engine?

[IanH755]

other than cats in the downpipes, any other cats on C6s?

I'm thinking about replacing the OEM downpipes with aftermarket exhaust and wonder how to make it 'road legal' (so that it can pass the MOT emissions at least)?

You have two choices, you can either keep a CAT in the downpipes which tends to be very expensive (£2000+ for a set) or keep a CAT in the exhaust but make it flow better.

I chose to have Decatted downpipes which was much cheaper (<£500) and easier to fit, and then I just added small 200cell Cats into the exhaust where the OEM "secondary cat" ones would be as shown below. After this photo was taken I added flanges to the CATS to make them removable so I could swap them out for track/road and refit them for MOT etc. With the 200cell cats fitted it passed an MOT no problem -

I wonder if downpipe replacement / swap /alteration is a doable task w/o dropping the engine?

Yes but it it risky. You drop the engine subframe for access but you risk breaking the studs on the turbo housing during downpipe removal (there is 1 per side which is particularly susceptible, can';t remember which) and if that snaps the engine HAS to be removed at that point. However many have swapped over the downpipes like this without issue so it's not a guarantee that things will break but just be aware that its possible and have the cash to one side just in case.

[cammmy]

That's a good idea. Any idea what it's worth to power/torque going down that route?

[Fatneck73]

So I have an MTM Centre Res delete and a Miltek catbak (Miltek done first)...and I like the sound....but more is always better...so Ian, I assume I can get done to mine (remove the rear boxes entirely) the same as you’d done????

[IanH755]

That's a good idea. Any idea what it's worth to power/torque going down that route?

I was running 900+ when I had these done but I'd never had it dyno'd with these fitted for an accurate answer so a "Butt-Dyno Guess" is about all I can give you but, as these are much more free flowing than the OEM ones (less restriction) and I'd de-catted the downpipes (again less restrictions) it didn't feel different enough to make a noticeable difference other than to the exhaust noise, which was a little more muted. Now it still could be a 10-50hp loss or it could be no loss at all, I don't know, but with 900+ I just couldn't feel much difference at all so whatever effect it had on my car was fairly minimal.

I got my CAT's from Arlows in Germany - https://www.arlows.de/Catalytic-Converter:::74.html

So I have an MTM Centre Res delete and a Miltek catbak (Miltek done first)...and I like the sound....but more is always better...so Ian, I assume I can get done to mine (remove the rear boxes entirely) the same as you’d done????

You could, but just be aware that volume change is pretty substantial so if you can make your back-boxes refittable (like I did with flanges) to keep others happy then I would highly recommend you do that! The hardest big of swapping the OEM boxes on/off was the lack of space for the back-box to slide off the rubber hanger, the Milltek ones are smaller so should be less of an issue.

[cammmy]

The simplest compromise would be exhaust cutouts that open under boost. Quieter startup and cruise but full noise when you step on the loud pedal. It's something I will probably do one day but I have other projects ahead in the queue.

Okay, thanks Ian. I'm sure the engine will have to come out one day, so I might look at doing something like your setup then.